with the recovered primaries. To facilitate this task we compute
a three-dimensional lookup table that implements the inverse of
Eq. (1), i.e., find αc,αm,αy given crgb. Note that this lookup table
has entries for both in-gamut and out-of-gamut colors, since
the scanned input images always contain some out-of-gamut pixels
(e.g., many of the pixels covered by black ink will be out of gamut).